1. Field of the Invention
This invention relates to an improvement in a control device for a commutatorless motor.
2. Description of the Prior Art
A commutatorless motor is generally comprised of a combination of a synchronous motor and a thyristor power converter. The commutatorless motor is hereinafter referred to as a thyristor motor.
In the thyristor motor in general, the phase of the armature current is advanced before that of the terminal voltage of the thyristor motor driven at high speed, so that commutation of the thyristor power converter, i.e., load commutation of the frequency converter is effected by the induced electromotive force of the synchronous motor. The above-mentioned phase difference between the armature current and the terminal voltage of the thyristor motor is called an advance angle .beta..
At low motor speeds where the induced electromotive force of the motor is small, load commutation is difficult and therefore power commutation or other means of commutation is effected. In this case, the advance angle .beta. is set at a small value in order to obtain a large motor torque. At high motor speeds, on the other hand, the advance angle .beta. must be set at a high value in order to effect the load commutation.
The value of the advance angle .beta. is thus required to be switched or changed between high and low speeds. The motor torque .tau. is generally expressed as .tau.=K.multidot.cos .beta. . . . . . (1). As apparent from the equation (1), if the advance angle .beta. is switched stepwise, the torque generated by the motor is changed suddenly, thereby adversely affecting the motor or load or, in some cases, resulting in commutation failure. In order to avoid this, it is common practice to reduce the armature current to zero temporarily so that commutation failure may be avoided and at the same time the torque change may be maintained at a small value. When the armature current is reduced to zero temporarily, the motor torque is also reduced to zero temporarily. Therefore the fact remains that a torque change occurs. Further, if thyristor motor is used for rolling mill or the like which is rapidly accelerated or decelerated, even the time (5 to 10 ms) required for a switching of the advance angle cannot be ignored. With the intention of obviating the problem of sudden change in motor torque, the present applicant has formerly proposed a control method for effectively developing motor torque by changing the advance angle continuously in accordance with motor speed. Such a method is disclosed in U.S. Pat. No. 3,894,277 and will be briefly explained below.
As shown in FIG. 1, when the motor is running at low speed below S.sub.1 at the time of starting, the advance angle .beta. is controlled at the small value of .beta..sub.1 in order to obtain a large motor torque. During the period where the advance angle .beta. changes from .beta..sub.1 to .beta..sub.2 in a high speed range beyond S.sub.2, the advance angle .beta. is increased along a straight line with the gradient shown in FIG. 1 in accordance with the motor speed. If the advance angle .beta. is controlled in this way, motor torque does not change very suddenly.
In some cases, however, the thyristor motor is required to be accelerated or decelerated at much higher rate than in the case mentioned above. In the case of a motor used for driving the rollers of a rolling mill, for instance, a predetermined high speed must be reached about two seconds after starting. Such a motor is required to be decelerated at a similarly rapid rate. If the advance angle .beta. is switched or changed within a possible speed range for such a sudden acceleration or deceleration, the inconveniences as mentioned below occur.
A tacho-generator is generally used for a speed detection. The low speed output characteristics of the tacho-generator are not linear and contain many ripples. As a result, in spite of a fixed rate of acceleration or deceleration, the advance angle .beta. variously changes within the range hatched in FIG. 2. A change in a predetermined pattern is thus not expected. In an extreme case, the advance angle .beta. changes in a pattern shown by dashed line D in FIG. 2. The advance angle .beta. rising along the sharp dashed line D changes quite suddenly. The result is a very abrupt change in torque, thus adversely affecting the motor and the load unavoidably.